/Engine/Source/Runtime/GeometryCore/Public/Selections/MeshFaceSelection.h
Grow selection outwards from seed triangles, until it hits boundaries defined by triangle and edge filters. Edge filter is not effective unless it (possibly combined w/ triangle filter) defines closed loops.
void FloodFill ( const [TArray](API\Runtime\Core\Containers\TArray)< int > & Seeds, const [TUniqueFunction](API\Runtime\Core\Templates\TUniqueFunction)< bool(int)> & TriFilterF, const [TUniqueFunction](API\Runtime\Core\Templates\TUniqueFunction)< bool(int)> & EdgeFilterF )